Generally, a base station or a relay station (hereinafter collectively referred to as “base station”) of a mobile communication system has been divided into a base station main unit configured to process transmit and receive signals and an antenna device including a plurality of radiating elements for transmitting/receiving radio signals. The base station main unit is usually installed at a low position on the ground, and the antenna device is installed at a high position such as a building roof or a tower, and the base station main unit and the antenna device may be connected to each other through a feeding cable or the like.
The antenna device of the base station includes a plurality of radiating elements for transmitting/receiving high frequency radio signals. At this time, the radiating elements are installed on one surface (for example, the front surface) of a metal reflecting plate having a relatively large area. Further, the antenna device is provided with various circuits or devices for processing signals transmitted and received via the plurality of radiating elements, and, for example, the circuits or devices may be fixedly installed on the back face of the reflecting plate. Such devices may include a phase shifter for adjusting the phase of the signals of the multiple radiating elements, and an apparatus for distributing/combining the transmit and receive signals of the internal devices and the radiating elements. At this time, the respective devices may be connected via a signal transmission line, and a substantial portion of which is formed of coaxial cable.
Meanwhile, in the antenna device of such a structure, it is a serious matter, in addition to having the required transmission/reception properties of the signal for the band, to suppress PIMD (Passive Intermodulation Distortion) generated by discontinuous contact surfaces, unstable connection state or other causes in the respective device structure itself and at the connections between devices.
In this case, the antenna device almost invariably requires a signal distributing/combining apparatus and the structure for making connections through soldering a coaxial cable to that signal distributing/combining apparatus, which in turn requires effective measures to suppress PIMD that can be generated due to unstable connection state occurring at the connections between the signal distributing/combining apparatus and the coaxial cable or due to nonuniform soldering state, etc.